> ## Documentation Index
> Fetch the complete documentation index at: https://www.plain.com/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# importThreadMessages

> Backfill up to 25 historical messages onto an existing thread. Each message requires an externalId for idempotency — re-importing the same ID returns NOOP for that message. INBOUND messages must be authored by a customer; OUTBOUND and NOTE messages must be authored by a user. Requires the thread:import permission.